In the last decade, a numerical control (NC) system for controlling a machining feed required to achieve a desired machining geometry have been increasingly adopted, not only for conventional milling, grinding and other machines but especially in combination with more sophisticated, electroerosion or electrical machining techniques, e.g. electrical discharge machining (EDM), electrochemical machining (ECM), electrochemical-discharge machining (ECDM), electrochemical grinding (ECG) and electrochemical-mechanical grinding (ECMG). This increased, more sophisticated adoption of NC systems is represented by the today's ever wide-spread use in the industry of highly automated traveling-wire (TW) and other "generic electrode" EDM machines capable of machining a die, mold or like highly intricate or "difficult to machine" article more efficiently than ever before. While an NC system may also be employed for sinking-type electroerosion (e.g. EDM) machines utilizing a formed tool electrode, the electrical machining (e.g. EDM) capabilities have been largely expanded in the form of "generic electrode" electroerosion (e.g. EDM) machining equipped with an NC unit. In these machines, the tool electrode is a wire, tape, cylindrical rod or like simple electrode and any be used to impart a complex shape to a workpiece with due precision.
Dies, molds or like articles as referred to have their own severe specifications to meet their particular purposes so that each must be prepared from a relatively expensive blank material and yet machined individually with an extremely high degree of machining precision. Furthermore, an extensive machining time ranging from an hour to days is needed to finish generating a required complete shape from the blank material. Such a die, mold or like article which can be efficiently machined with a numerically controlled (NC) machine, especially but not exclusively electroerosion machine, is thus of high value and price, any accidental machining error during its course of machining is of vital problem.
In an NC machine (e.g. EDM machine), the assigned function of the NC system is to effect a machining feed required to follow stock removal at the machining interface (e g. discharge erosion at the EDM gap) and to enable the stock removal (e.g. discharge erosion) to continue precisely along a programmed path. The machining feed is effected by means of a succession of drive pulses based upon digital commands programmed on a record medium so that the desired relative advancement may be performed as accurately as possible in accordance with the programmed commands. Motor means such as a DC motor or stepping motor for energization by the drive pulses is drivingly coupled to a drive member such as a leadscrew which is in turn connected to support means for the movable tool electrode or workpiece. Each individual drive pulse is, for the sake of precision, typically designed to effect an increment of the relative advancement as small as 1 .mu.m or less, and is furnished over a fraction of a second. Such successive drive pulses must be consecutively furnished to the motor means throughout the prolonged machining operation as mentioned above to continue the required machining feed precisely along the programmed feed path.
Such drive systems, however, commonly entail conversion of electrical to mechanical signals and further mechanical conversion of rotary to longitudinal signals or displacements. Thus, it has been recognized that mechanical `error` may develop in the stages of conversion due, for example, to an error in the lead or pitch of a lead screw and a backlash in various components. Furthermore, of even greater importance is the discovery that changes in gap conditions and machining parameters including tool or electrode geometry may prevent each NC command from being precisely reflected in an actual amount of material removed and hence on the corresponding machining feed or relative displacement. As a result, it is possible that minor deviations occurring from time to time for one or more of these various causes will accumulate to result in a serious machining error in the workpiece and a consequent irreparable damage thereof.